1. Field of the Invention
The present invention is generally related to photolithography systems, and more particularly, to maintaining an oxygen-purged optical path through a reticle and pellicle.
2. Related Art
In the fabrication of integrated circuits, photolithographic and projection printing techniques are used. In photolithography, an image contained on a reticle is projected onto a wafer having a photosensitive resist thereon. The reticle or mask is used to transfer a desired image onto the silicon wafer. The semiconductor wafer surface is coated with photosensitive resist so that an image is etched thereon. A pellicle may be used in combination with the reticle to protect the reticle surface from damage. The pellicle is traditionally mounted on a solid frame to the reticle.
Some wavelengths of light used in photolithography are sensitive to absorption by atmospheric oxygen. Hence, when such oxygen-sensitive light wavelengths are used in photolithography, they must be transmitted through an oxygen-purged atmosphere.
A photolithography system is typically located in a clean room environment. In some situations, the ambient atmosphere of the clean room cannot be purged of oxygen because this may cause other problems with the photolithography process. For instance, a laser interferometer used in a lithography system may be sensitive to changes in the index of refraction of the air, which may occur with a change to an oxygen-free atmosphere. Hence, the oxygen-free environment may have to be restricted to less than the entire lithography system. What is needed is a transmission medium for light wavelengths that have high absorption in an oxygen-containing environment.
A pellicle is generally mounted on a frame opposite a corresponding reticle. Hence, an air gap may exist between the reticle and pellicle. What is needed is a transmission medium through the reticle-to-pellicle air gap for light wavelengths that have high absorption in an oxygen-containing environment.
The present invention is directed to a method and apparatus for a reticle with a purged pellicle-to-reticle gap. The present invention maintains a substantially oxygen-free, purge gas environment in a pellicle-to-reticle gap. The purge gas environment provides a transmission medium for light wavelengths that have high absorption in a non-purged environment.
In a preferred embodiment, the present invention is applied to a photolithography system. A porous frame between a reticle and a pellicle creates a gap or space between the reticle and pellicle. The porous frame may passively filter ambient air entering the gap through the porous frame to create a substantially particle-free gap. The particulate protection is required to ensure that particles do not deposit on the critical reticle surface, degrading the reticle image projected onto a semiconductor wafer surface. This includes protection during storage of the reticle and usage of the reticle in a lithographic process.
The passive or static porous frame acts to normalize the pressure within the reticle to pellicle gap with the external ambient air atmosphere. This normalization action effectively reduces or eliminates distortion of either the reticle and/or pellicle due to atmospheric pressure.
The porous frame includes a first opposing surface with a first opening. The first opposing surface is configured to mate with the pellicle. The porous frame includes a second opposing surface with a second opening. The second opposing surface is configured to mate with the reticle to enclose the optical gap between the pellicle and the reticle.
A purged reticle to pellicle gap may be formed by filling the gap with a purge gas that does not contain oxygen. The purge gas in the gap may be maintained dynamically by continuously infusing the purge gas.
A dynamic porous frame may be coupled to a purge gas supply. The purge gas supply inserts a purge gas into the gap between the reticle and pellicle through the porous frame, establishing a purge gas flow in the gap within the porous frame.
A vacuum source may be coupled to the dynamic porous frame to remove gas from the reticle-to-pellicle gap environment through the porous frame, further providing continuous gas flow in the reticle.
The purge gas flow in the gap of a dynamic porous frame may be balanced with an external atmospheric pressure to reduce or eliminate reticle or pellicle distortions.
The porous frame of the present invention is applicable to other environments, including other optical environments. In an example alternative optical embodiment, the porous frame can provide a purged optical path between any optical source surface and any optical target surface. The optical source surface and optical target surface may be any suitable optical surfaces known to persons skilled in the relevant art(s).
Further embodiments, features, and advantages of the present inventions, as well as the structure and operation of the various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.